Developing powder in which the particles have a charge opposite to that of the charge image to be developed is frequently used for developing electrostatic charge images formed on a suitable image forming medium in electrophotographic or electrostatic image forming processes. Normally, the charge on the powder particles is obtained by tribo-electric charging against a part of the developing device or against carrier particles mixed with the developing powder. Lack of uniformity frequently occurs in the development of large solid image areas with such developing powder. Such non-uniformity can be due to various reasons, such as vibration and impacts in the drive system for the image forming medium and/or the developing device, exhaustion of the stock of developing powder on the developing means, and divergence of the outgoing electric field in the case of large solid image areas (known as "edge field effects").
When such a non-uniformly developed image is transferred in an electric field (as is conventional) to a receiving material and then fixed thereon, the result is an image area with a non-uniform optical density. Typically, the edges have a greater optical density than the central part of the area. In the case of development with black developing powder this defect is less visible than in color developing where the defect is very distracting. Accordingly, it is an object of the invention to provide a means and method for reducing or eliminating this defect.
It has surprisingly been found that the visual defect can be eliminated by the application of an electric field across the image forming medium and the image receiving medium in a direction such that the electrostatically charged developing powder is subjected to a force directed towards the image forming medium during pressure contact between the two mediums. Although the pressure adhesion transfer step is already known from U.S. Pat. No. 3,591,276, it has been found that the combination of adhesion transfer and a counteracting electric field gives the surprising result that a uniform thickness of the developing powder layer is obtained on the image receiving medium. The use of an electric field to transfer a toner image from an image forming medium to an image receiving medium is very well known for instance from U.S. Pat. No. 3,734,724, it discloses a conventional transfer method where developed images are transferred by an electric field from a transparent photoconductive element, e.g.. conductive film, to a metal receiving medium, e.g.. lithographic plate or nonmetallic medium wherein an electrical repelling potential is applied to the receiving medium, just before the transfer zone to prevent premature transfer of toner particles.
In Japanese Application 59-50474 a supporting electric field is applied, directing from the image forming medium to a receiving belt, to improve the transfer rate of the pressure transfer step.